.\" Copyright (c) 1996 Tom Bjorkholm <tomb@mydata.se>

.\"

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.\" 1996-04-11 Tom Bjorkholm <tomb@mydata.se>

.\"            First version written (1.3.86)

.\" 1996-04-12 Tom Bjorkholm <tomb@mydata.se>

.\"            Update for Linux 1.3.87 and later

.\" 2005-10-11 mtk: Added NOTES for MREMAP_FIXED; revised EINVAL text.

.\"

.TH MREMAP 2 2017-09-25 "Linux" "Linux Programmer's Manual"

.SH NAME

mremap \- remap a virtual memory address

.SH SYNOPSIS

.nf

.BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"

.B #include <sys/mman.h>

.PP

.BI "void *mremap(void *" old_address ", size_t " old_size ,

.BI "             size_t " new_size ", int " flags ", ... /* void *" new_address " */);"

.fi

.SH DESCRIPTION

.BR mremap ()

expands (or shrinks) an existing memory mapping, potentially

moving it at the same time (controlled by the \fIflags\fP argument and

the available virtual address space).

.PP

\fIold_address\fP is the old address of the virtual memory block that you

want to expand (or shrink).

Note that \fIold_address\fP has to be page

aligned.

\fIold_size\fP is the old size of the

virtual memory block.

\fInew_size\fP is the requested size of the

virtual memory block after the resize.

An optional fifth argument,

.IR new_address ,

may be provided; see the description of

.B MREMAP_FIXED

below.

.PP

If the value of \fIold_size\fP is zero, and \fIold_address\fP refers to

a shareable mapping (see

.BR mmap (2)

.BR MAP_SHARED ),

then

.BR mremap ()

will create a new mapping of the same pages.

\fInew_size\fP

will be the size of the new mapping and the location of the new mapping

may be specified with \fInew_address\fP; see the description of

.B MREMAP_FIXED

below.

If a new mapping is requested via this method, then the

.B MREMAP_MAYMOVE

flag must also be specified.

.PP

In Linux the memory is divided into pages.

A user process has (one or)

several linear virtual memory segments.

Each virtual memory segment has one

or more mappings to real memory pages (in the page table).

Each virtual memory segment has its own

protection (access rights), which may cause

a segmentation violation if the memory is accessed incorrectly (e.g.,

writing to a read-only segment).

Accessing virtual memory outside of the

segments will also cause a segmentation violation.

.PP

.BR mremap ()

uses the Linux page table scheme.

.BR mremap ()

changes the

mapping between virtual addresses and memory pages.

This can be used to implement a very efficient

.BR realloc (3).

.PP

The \fIflags\fP bit-mask argument may be 0, or include the following flag:

.TP

.B MREMAP_MAYMOVE

By default, if there is not sufficient space to expand a mapping

at its current location, then

.BR mremap ()

fails.

If this flag is specified, then the kernel is permitted to

relocate the mapping to a new virtual address, if necessary.

If the mapping is relocated,

then absolute pointers into the old mapping location

become invalid (offsets relative to the starting address of

the mapping should be employed).

.TP

.BR MREMAP_FIXED " (since Linux 2.3.31)"

This flag serves a similar purpose to the

.B MAP_FIXED

flag of

.BR mmap (2).

If this flag is specified, then

.BR mremap ()

accepts a fifth argument,

.IR "void\ *new_address" ,

which specifies a page-aligned address to which the mapping must

be moved.

Any previous mapping at the address range specified by

.I new_address

and

.I new_size

is unmapped.

If

.B MREMAP_FIXED

is specified, then

.B MREMAP_MAYMOVE

must also be specified.

.PP

If the memory segment specified by

.I old_address

and

.I old_size

is locked (using

.BR mlock (2)

or similar), then this lock is maintained when the segment is

resized and/or relocated.

As a consequence, the amount of memory locked by the process may change.

.SH RETURN VALUE

On success

.BR mremap ()

returns a pointer to the new virtual memory area.

On error, the value

.B MAP_FAILED

(that is, \fI(void\ *)\ \-1\fP) is returned,

and \fIerrno\fP is set appropriately.

.SH ERRORS

.TP

.B EAGAIN

The caller tried to expand a memory segment that is locked,

but this was not possible without exceeding the

.B RLIMIT_MEMLOCK

resource limit.

.TP

.B EFAULT

"Segmentation fault." Some address in the range

\fIold_address\fP to \fIold_address\fP+\fIold_size\fP is an invalid

virtual memory address for this process.

You can also get

.B EFAULT

even if there exist mappings that cover the

whole address space requested, but those mappings are of different types.

.TP

.B EINVAL

An invalid argument was given.

Possible causes are:

.RS

.IP * 3

\fIold_address\fP was not

page aligned;

.IP *

a value other than

.B MREMAP_MAYMOVE

or

.B MREMAP_FIXED

was specified in

.IR flags ;

.IP *

.I new_size

was zero;

.IP *

.I new_size

or

.I new_address

was invalid;

.IP *

the new address range specified by

.I new_address

and

.I new_size

overlapped the old address range specified by

.I old_address

and

.IR old_size ;

.IP *

.B MREMAP_FIXED

was specified without also specifying

.BR MREMAP_MAYMOVE ;

.IP *

\fIold_size\fP was zero and \fIold_address\fP does not refer to a

shareable mapping (but see BUGS);

.IP *

\fIold_size\fP was zero and the

.BR MREMAP_MAYMOVE

flag was not specified.

.RE

.TP

.B ENOMEM

The memory area cannot be expanded at the current virtual address, and the

.B MREMAP_MAYMOVE

flag is not set in \fIflags\fP.

Or, there is not enough (virtual) memory available.

.SH CONFORMING TO

This call is Linux-specific, and should not be used in programs

intended to be portable.

.\" 4.2BSD had a (never actually implemented)

.\" .BR mremap (2)

.\" call with completely different semantics.

.SH NOTES

Prior to version 2.4, glibc did not expose the definition of

.BR MREMAP_FIXED ,

and the prototype for

.BR mremap ()

did not allow for the

.I new_address

argument.

.PP

If

.BR mremap ()

is used to move or expand an area locked with

.BR mlock (2)

or equivalent, the

.BR mremap ()

call will make a best effort to populate the new area but will not fail

with

.B ENOMEM

if the area cannot be populated.

.SH BUGS

Before Linux 4.14,

if

.I old_size

was zero and the mapping referred to by

.I old_address

was a private mapping

.RB ( mmap "(2) " MAP_PRIVATE ),

.BR mremap ()

created a new private mapping unrelated to the original mapping.

This behavior was unintended

and probably unexpected in user-space applications

(since the intention of

.BR mremap ()

is to create a new mapping based on the original mapping).

Since Linux 4.14,

.\" commit dba58d3b8c5045ad89c1c95d33d01451e3964db7

.BR mremap ()

fails with the error

.B EINVAL

in this scenario.

.SH SEE ALSO

.BR brk (2),

.BR getpagesize (2),

.BR getrlimit (2),

.BR mlock (2),

.BR mmap (2),

.BR sbrk (2),

.BR malloc (3),

.BR realloc (3)

.PP

Your favorite text book on operating systems

for more information on paged memory

(e.g., \fIModern Operating Systems\fP by Andrew S. Tanenbaum,

\fIInside Linux\fP by Randolf Bentson,

\fIThe Design of the UNIX Operating System\fP by Maurice J. Bach)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.